Self-biased analog circuits may have the potential to enter undesirable non-linear operating regions at low supply voltage. These undesirable operating regions can be undetectable during normal function and may result in circuit failure.
More specifically, self-biased analog circuits can include three regions of operation. By way of example, Region 1 is defined as a functional state characterized by an operational amplifier output voltage (Vampout) at Analog Power (VDD) and approximately zero current flow in the bandgap reference legs. Region 2 is defined as a functional state characterized by a Vampout at Analog Ground (GND) and high current flow in the bandgap reference legs. Region 2 is unique to self-biased analog circuits. Regions 1 and 2 are the undesirable non-linear operating regions.
A start-up circuit initializes an analog circuit loop into a desired operating region during a power-on sequence. However, if the power supply headroom is too low when a start-up circuit is engaged, only the two undesirable operating regions, i.e., Region 1 and Region 2 may exist.
However, as the power supply headroom is increased, a third operating region, Region 3, is created between Region 1 and Region 2. Region 3 is the desired linear operating region of the bandgap reference where the output voltage Vten is independent of process, temperature, or supply voltage. Thus, in Region 3 Vampout is in a range between VDD and GND. The magnitude of the ranges of voltages that define Regions 1 and 2 are fixed. Thus, as the total power supply voltage is decreased, the voltage range of Region 3 is decreased. Conversely, as the total power supply voltage is increased, the voltage range of Region 3 is increased, while the ranges of the voltages that define Regions 1 and 2 remain the same.
If enabled, a conventional start-up circuit prevents operation of a self-biased analog circuit in undesirable non-linear Region 1. Thus, the conventional start-up circuit will enable start-up of the self-biased circuit by pulling Vampout from VDD towards GND.
However, the conventional start-up circuit may not prevent operation of the self-biased analog circuit in undesirable non-linear Region 2. Rather, if the power supply headroom is too low when the start-up circuit is engaged, such that only Region 1 and Region 2 exist, the self-biased circuit may pull Vampout from the undesirable non-linear Region 1 directly into the undesirable non-linear Region 2. Additionally, if the self-biased analog circuit enters Region 2 during the power-on sequence, the circuit may tend to stay in that region because of first and second positive feedback loops, described further below.